This invention relates to the halogenation of aromatic compounds. More particularly, it relates to chlorination methods and catalyst compositions capable of producing predominantly para-chloro aromatic compounds.
Chlorination of aromatic compounds such as toluene and xylenes is a known reaction affording useful compounds. The most useful of these compounds for many purposes are the para-chloro aromatic compounds. para-Chlorotoluene, for example, is an intermediate capable of conversion into many useful chemicals. para-Chloro-ortho-xylene (also sometimes referred to as 4-chloro-1,2-dimethylbenzene) is another useful compound, which can be oxidized to 4-chlorophthalic acid, which is in turn an important intermediate in the production of polyetherimides. However, the production of these useful para-chloroaromatic compounds is complicated by the simultaneous production of numerous undesirable by-products. Thus, chlorination of toluene and xylenes, such as ortho-xylene produces the para-monochloro isomer in admixture with other isomers, such as ortho-chlorotoluene and 3-chloro-1,2-dimethylbenzene, respectively. In addition, numerous polychlorinated products are also generally produced.
Many of the known methods for chlorination of aromatic compounds involve reaction with elemental chlorine in the presence of Lewis acids, such as for example, ferric chloride, antimony trichloride, antimony pentachloride, zinc chloride and aluminum chloride, which are also generally used as catalysts in Friedel-Crafts reactions, such as alkylation and acylation.
However, the use of such catalysts generally does not lead to enhanced selectivity for the desired para-chloroaromatic isomer and minimized formation of polychlorinated products. Various publications, including many U.S. patents, go further in describing mixed catalyst systems in which another catalyst component is an organosulfur compound. The organosulfur compounds disclosed in these publications are of very diverse structures. Some examples of organosulfur compounds that have been used include phenoxathiins, thianthrenes, and phenothiazines. Illustrative patents are U.S. Pat. Nos. 3,226,447, 4,031,142, 4,031,145, 4,031,147, 4,190,609, 4,250,122, 4,289,916, 4,647,709, 4,925,994, and 5,210,343; and European Patent Application No. 126669. Progress in the field of para halogenation of aromatic compounds notwithstanding, there remains a strong need to develop further improvements both in terms of product yield and selectivity.